SANTA CLARA — Minutes after officially signing with the 49ers on Monday, running back DeShaun Foster sounded bored stiff. He talked in a low monotone with answers that sometimes ranged from “yes” to “no.”

Is this really where he wanted to be?

“I’m kind of quiet,” Foster said, finally laughing. “That’s just me.”

His demeanor is fine with the 49ers, who signed him to run the ball — not his mouth. Foster, a six-year veteran, comes aboard as the backup to workhorse running back Frank Gore.

The 49ers also formally announced the signings of linebacker Dontarrious Thomas and quarterback J.T. O’Sullivan.

Foster, 28, a second-round pick out of UCLA in 2002, averaged 53 rushing yards a game during his Carolina Panthers career.

After getting cut two weeks ago, Foster looked around for a starting job but discovered almost every team was set at his position. He said he decided on the 49ers because he considers them a team on the rise.

“They have a lot of young talent on the team, with veteran receivers. I think that the sky is the limit with this offense,” said Foster, who is listed at 6-foot, 222 pounds.

New offensive coordinator Mike Martz has a reputation for emphasizing the passing game, but Foster said he expects enough balance to stay busy. He is also a big fan of Gore, who has 572 of the 49ers’ 796 rushing attempts — 72 percent — over the past two seasons. When asked what his role would be, Foster responded: “Just basically to get in there and spot Frank.”

Foster has at least 800 rushing yards over the past three seasons. He is also responsible for the two longest runs in Panthers history: a 71-yarder in 2004 and a 70-yarder in ’06.

Though he struggled with injuries early in his career — a knee injury wiped out his rookie season in 2002 — Foster played in all 16 games last season and rushed for 876 yards. The Panthers, however, tired of his ball-security problems. Foster lost seven of his 11 fumbles over the past two seasons.

Notes: Thomas is in line to take over for Derek Smith as the strong inside linebacker — the so-called “ted” position in the 49ers’ 3-4 scheme. Thomas, 27, played 59 games over the four previous seasons with the Minnesota Vikings. … O’Sullivan, a former UC Davis standout who backed up in Martz’s offense with the Detroit Lions last season, is in line to be the third quarterback. … The 49ers are hosting open auditions for their Gold Rush dance team March 30 at Stanford. Information is available at 49ers.com.

Daniel Brown is a sports writer for the Bay Area News Group known for his award-winning profiles on subjects ranging from Jerry Rice to Sergio Romo to Taylor Swift. The Cotati native is a graduate of UC Davis.